This invention relates generally to systems for removing water from an oil emulsion by electrostatically promoting coalescence of water droplets into sizes large enough to gravitate from the oil. More specifically this invention relates to an improved electrostatic coalescence system for separating a dispersed water phase from a continuous oil phase in which independent AC and DC electric fields are employed together with hydrophilic electrodes to take advantage of a number of phase separation mechanisms to accomplish more complete phase separation.
In the petroleum industry electrostatic coalescence techniques have been employed for resolving water-in-oil emulsion. Generally, an AC electric field is used when the electrical conductivity of the continuous oil phase is between 10.sup.-8 to 10.sup.-10 ohm.sup.-1 cm.sup.-1, and a DC electric field is used when the electrical conductivity of oil is less than 10.sup.-10 ohm.sup.-1 cm.sup.-1. The conductivity is generally dependent on the amount of water in the crude oil.
Not only is water present naturally in crude oil, but water is added and vigorously mixed with crude stock to aid in the process of desalting of the crude prior to refining. Desalting is a process in which salts, water and solid particles are removed from the crude oil through the separation of the salt and solid-laden water phase from the continuous oil phase. The presence of salts, water and solids in crude causes various problems in the operation of an oil refinery. It causes the formation of sludge in crude storage tanks which reduces the storage capacity. Sludge may also accumulate in distillation columns and cause serious operation problems. Salt deposition in equipment such as heat exchanger tubes in crude furnaces leads to plugging and fouling of the process equipment. Salts also catalyze the formation of coke in the furnace. Coke deposition reduces the heat transfer coefficient, causing localized heating and eventually leading to blistering or rupture of the tubes.
Further, salts in crude oil cause corrosion because they lead to evolution of hydrochloric acid in distillation equipment. The severe corrosion from the combination of sulfur-containing compounds with hydrogen chloride, is caused by oxidation reduction reactions. The units most susceptible to corrosion from hydrogen chloride are those where moisture may be present, such as in condensers.
When the crude feed contains salts, the solubility and ductility of asphalt products can be affected. Salts also cause fuel oil to have excessively high ash content. Gas turbine fuel must be salt-free, otherwise fused salts can seriously damage the turbine blades.
Thus, it will be seen that salts in crude oil must be removed prior to charge into crude distillation units. Water and solid particles are also removed as the result of extraction of salts with water. It has been recognized in the art that the fastest method of water extraction from crude feed is through the use of electrostatic coalescence methods.
Generally, systems employing electrostatic coalescence methods have used either DC or AC electric fields only. One system is known in which a pulsed DC field is employed to simulate a combination AC and DC coalescing system. Details of this system may be had by referring to the following patents:
1. U.S. Pat. No. 3,772,180 issued to Floyd L. Prestridge for "Electric Treater," Nov. 13, 1973.
2. U.S. Pat. No. 3,847,775 issued to Floyd L. Prestridge for "Process for Electrical Coalescing of Water," Nov. 12, 1974.
Briefly, the referenced system employs a single set of electrodes mounted above the emulsion-water interface in the treater containment. The positive and negative electrodes are alternately pulsed to produce a generally uniform DC field between the electrodes and a simulated non-uniform AC field between the electrodes and the interface which acts as a ground electrode. The field gradients vary depending upon the dispersed phase volume fraction of the emulsion introduced at the interface. Thus, since the AC and DC fields are produced using common electrodes, the dispersed phase volume entering the DC field region controls both the AC and DC field components. Non-uniformity in the fields can cause very high localized electric field gradients which produce water droplet breakup, redispersing the water phase and reducing the efficiency of the unit. Further, the AC and DC fields cannot be independently controlled for selected AC and DC electric field gradients for most efficient operation. The DC electrode spacing is dictated by spacing requirements for producing the simulated AC field. For more complete phase separation it has been found that the spacing between the DC electrodes, through which the continuous oil phase passes after the bulk of the water is removed by the AC electrodes, should be small to provide short drop residence time required for the removal of small dispersed water drops from the continuous oil phase. This allows collection of small drops of water at hydrophilic electrodes which is the primary DC phase separation mechanism of the present invention. Drop-drop coalescence, settling of coalesced drops, and drop contact charging are secondary phase separating mechanisms provided by the independent DC electrode arrangement contemplated by the present invention.
Therefore, it is an object of the present invention to provide an improved electrostatic coalescence unit for removal of water dispersed in crude oil with independent AC and DC electrode structures which provides more complete separation.
Yet another object of this invention is to provide an electrostatic coalescence unit as set forth in the above object in which multiple-phase separation mechanisms are utilized to enhance more complete phase separations.
Other objects and many of the attendant advantages of the present invention will be obvious to those skilled in the art from the detailed description of the invention which follows hereinbelow.